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Has Canberra's weather changed according to Bureau of Meteorology records?

By Aarti Betigeri, Monday August 21, 2017 - 17:01 EST
ABC licensed image
We've received a lot of questions about Canberra's weather. - ABC licensed

The weather is an ongoing topic of conversation for Canberrans and out-of-towners alike, who struggle to deal with its sub-zero temperatures in winter.

But as spring approaches, the mercury is definitely on the way back up, even as early as August, when daffodils start to bloom.

Lifelong Canberra resident Jacqui White strongly suspects that spring and autumn are both blowing in earlier than they did in the past.

She asked Curious Canberra: "I wanted to know if the weather has changed noticeably in the last few decades according to the Bureau of Meteorology?"

"When I was younger I recall [it] being quite warm in May whereas recently it seems to have been really cold in May and the opposite in August."

We get a lot of questions about the weather, so we decided to take Jacqui's query on.

To investigate whether the seasons are speeding up, I first took to the Capital Regions Farmers Market in Watson, to ask stallholders what they think.

On the topic of weather, they had mixed reports.

"We have seen a drop in rainfall ... it has been getting a little bit colder," said Wilson, a mushroom farmer from Murrumbateman.

"I do know it has been affecting other farmers as well, especially with the amount of water they're getting."

"This year especially the weather's been very cool, the nuts have responded by staying in good condition, and that's helped immensely," a chestnut farmer said.

But the problem with anecdotal evidence is that it varies depending on individual experience.

Here's what the BOM found

To answer Jacqui's question, I turned to Bureau of Meteorology climatologist Joel Lisonbee.

He came up with a few ways to measure whether things are changing.

First, he drew up a chart to map the first day after July 1 when the temperature reached 20 degrees at the weather station at Canberra Airport.

"You usually expect the first 20 degree day to happen in early September," Joel said.

"There have been plenty of years [when] that has happened in late August but on average early September, early spring, you expect that first day above 20 in Canberra to happen. So that['s] a good objective measure to look at."

Joel plotted the date every year, and drew a line through the results to see how things were changing.

And changing they are, the first day of above 20 degrees is happening a little earlier, but not a lot.

"Overall there's a trend upwards in our autumn and spring temperatures, autumn and spring are getting warmer, spring more so than autumn," he said.

"We're looking at an increase of about 0.2 degrees per decade for spring and about 0.13 degrees per decade for autumn.

"And these are trends that are consistent with the broad area of south-east Australia, we're seeing spring especially is getting warmer on average throughout our meteorological record."

Put differently, Joel explained that a temperature of 20 degrees or above was happening "one day per decade on average earlier now than in the 1940s."

Another graph showed the number of warm days in September, again showing a definite upward trend.

Now the methodology for calculating what constitutes a 'warm day' to a climatologist is actually somewhat complex.

Joel has taken the period from 1961 to 1990 and looked at the average daily September temperature throughout those years.

A 'warm day', he says, is one that exceeds the 90th percentile of those averages.

In the period Joel examined, on average there were 3.3 days every September that reached the 90th percentile.

"Whereas in the decade roughly 2006 to 2015, we're seeing 7.1 days that reach the 90th percentile for the number of daily maximum temperatures for Canberra," he said.

"So that's quite a dramatic change during that transition time."

Basically, we are getting warmer, earlier, and it appears that Jacqui's hunch, is indeed correct.

It's something that could be a little contentious, however. The issue of climate change has for years been on the agenda, but is heavily politicised.

So much so that a handful of individuals and organisations contacted to take part in this story refused to be drawn on whether they'd noticed any signs of warming weather in Canberra.

Jacqui also had a pretty specific question she wanted answered, regarding frost.

"It used to always be the standard not to plant your tomatoes before Melbourne Cup Day because the frost would kill them. But in the last couple of springs there's been pretty much no frosts come September," she said.

Melbourne Cup Day comes on the first Tuesday of November, when temperatures are in the low twenties.

Joel said that's a difficult one to answer, mostly because the official weather station was moved in 2008, meaning there's too much variation in minimum temperatures before and after that, to paint an accurate picture.

"Rule of thumb - when temperatures drop below two degrees you can get frost. It doesn't always apply but generally you can look at, when has the temperature dropped below two degrees," he said.

"In Canberra, we don't have a consistent record of minimum temperatures."

Who asked the question?

Jacqui White, who lives in the inner south, was born in Canberra, and while she lived elsewhere for long stints – England, New Zealand, Queensland, Bungendore – she kept returning to the capital.

In a former life she worked in admin and as a vet nurse; now it's horses and her children who take up most of her time. She loves Canberra, particularly for the environment,  as well as the opportunity to have a ring-side seat to the spectacle of federal politics.


© ABC 2017

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